Gravel pack material (granular material) is commonly used in well construction to stabilize the bore hole and provide filtration to prevent fine grain formation material from entering the well with the liquids or gasses produced. There is considerable advantage in providing a means of attaching a gravel pack filter to the well screen itself prior to the installation of the well screen in the bore hole, as the well driller has then assured that the well can be completed in a manner which will not produce formation material (sand).
There is an existing design which attaches the gravel pack material to the well screen by cementing granular material to the well screen (e.g.; "Aquastream", U.S. Pat. No. 4,624,314). This design results in a product which is somewhat fragile and may not be easily cleaned of clogging formation material or drill cutting. An improvement in that design is described in this application and is achieved by forming the outside shape of the granular envelope with a wave-like or bumpy surface with channels which allow free movement of well cleaning fluid around the outside of the screen, and having its thicker portions covering the slotted (open) sections of the well screen and its thinner parts at the places which are not slotted.
Another presently available well screen design uses granular material in the cavity between two telescoping well screens (Western K-L Associates, Inc. "Pre-Pac" dual wall well screen). This product is in some cases somewhat heavy and larger in diameter than desirable because of the need to use commonly available pipe sizes. Also, the open area available to transmit fluids into the well must be limited to that which can be produced by that slot size which is usable to retain the size of the gravel used in the gravel pack.
Another type of well screen now available uses two wire wound well screens (one inside the other) with a granular gravel pack between them (Johnson Filtration Systems: "Channel Pack (TM)"). This product was borrowed from existing petroleum production technology. The well screen produced is excessively expensive, is of a high shipping weight and its excessive diameter is a disadvantage.